Typical vertical bipolar transistors exhibit greater current gain in a direction downward from the top surface of the semiconductor into the bulk of the semiconductor, than the current gain in a direction upward from the bulk of the semiconductor to the surface. This asymmetry in vertical current gain is related to the ratio of the injected current collected by the collector region to the injected current recombined in the base region and which does not reach the collector.
One reason for this asymmetry in current gain arises from the area difference or ratio of the top junction area to the bottom junction area. Since the top surface of a typical vertical bipolar device must contain both the emitter and base contacts, the emitter area must necessarily be smaller than the collector area, which may fully occupy the opposite surface, in order to accommodate the base contact. As illustrated in FIGS. 1A and 1B, a greater number of carriers 54 injected by an emitter 52 disposed on a surface which includes the base 58, are collected than are collected when injected (60) by a region 56 occupying the entire surface. As illustrated in FIG. 1A, when the top emitter 52 is injecting carriers 54, the collection of the injected carriers by the collector 56 is relatively unaffected by the base regions 58. By contrast, when the carriers 60, 62 are injected by the collector 56 whose area is not diminished by other semiconductor regions, only those carriers 60 which are injected under the emitter 52 are collected by the emitter 52. The carriers 62 which are injected under the base 58 are recombined in the base 58, adding to the base current and thus lowering the current gain of the transistor 50. If the base 58 contacts are made as small as possible to minimize this effect, the base resistance increases, thus reducing other transistor performance characteristics.
A second reason for asymmetry in current gain arises from the difference in the doping of the top emitter and the underlying collector. The doping of the collector and emitter and the relative doping differential between adjacent N and P regions affects the ability of the emitter and collector to inject carriers. While the top-disposed emitter may be doped to the solid solubility limit of silicon by solid state diffusion, typically in the order of 10.sup.21 cm.sup.-3, the doping of the collector is limited to a lower concentration, typically 10.sup.19 cm.sup.-3 by the crystal growth process. The diffusion of dopant is graphically illustrated in FIG. 2 wherein the emitter region 52A is more highly doped than the collector region 56A. Process steps which enhance the concentration of dopant in the collector by solid state diffusion are diminished in subsequent process steps. The dopant concentration gradient is further degraded through sublimation, also known as "autodoping," during epitaxial growth of the lightly doped region. As a result, the concentration of dopant in the emitter 52 is typically several orders of magnitude higher than the collector, further contributing to the asymmetrical current gain of the vertical transistor.